1. Field of the Invention
The invention relates generally to vehicles that have low to zero turning radius capability. Zero turning radius vehicles are often described as ZTR vehicles. However, this name has also been used to described vehicles capable of a turning radius that is not precisely zero. More specifically, the invention relates to steering systems, steering and speed coordination systems, and vehicles that comprise one or both types of systems.
2. Description of Related Art
ZTR vehicles are generally propelled by rear drive wheels, which can be driven at different speeds to accomplish steering. The speed and direction of rotation of the drive wheels of some ZTR vehicles are controlled through separate hand levers. Some users find these levers confusing because they control both vehicle speed and direction.
Some ZTR vehicles use a steering wheel instead of separate control levers. However, some of these vehicles do not provide correct steering when the vehicle is in reverse. For example, when backing up and turning the steering wheel to make a left-hand turn, some of these vehicles produce a right-hand rear turn where the front of the vehicle—instead of the rear—moves to the left. See U.S. Pat. No RE 34,057 as an example of such a ZTR vehicle.
John Deere introduced a series of Spin-Steer Technology™ (SST) tractors. The SST tractors possess a rear-wheel driven differential steering system controlled by a steering wheel, and a vacuum-actuated reverse logic system that provides for conventional steering in reverse. The front wheels are caster wheels that are not steerable. See U.S. Pat. No. 6,256,357 for a description of these tractors.
U.S. Pat. No. 6,601,663 discloses a ZTR vehicle that utilizes a steering wheel to control steering, and a single hydraulic variable displacement pump and dual variable displacement hydraulic motors, each of which is coupled to a ground engaging wheel that is used to steer and drive the vehicle. This ZTR vehicle provides for proper steering in the forward and reverse directions.
U.S. Patent Application Publication No. 2003/0102171 also discloses a ZTR vehicle capable of proper wheel-effected steering in forward and reverse. The independently-actuated rear wheels drive the vehicle. They also steer the vehicle by rotating at different speeds and/or directions.
One problem with using caster wheels as non-steerable front wheels on ZTR vehicles is noticeable when driving on the side of a hill. Gravity will tend to pull the vehicle down the hill. This may cause the portion of the vehicle supported by the caster wheels to turn downhill against the operator's wishes. Additionally, when attempting to turn the ZTR vehicle uphill, the drive wheels may loose traction as the operator tries to produce the torque required to get the castor wheels pointed in the uphill direction.
Steerable front wheels have been used on ZTR vehicles. See U.S. Pat. No. 3,362,493 (Davis, et al.) and U.S. Pat. No. 5,042,238 and U.S. Patent Application Publication No. 2003/0019682. However, each has shortcomings. For example, the Davis patent device is not equipped with a system that can reduce the speed of the outboard drive wheel of a vehicle entering an extreme turn at a constant speed input.
U.S. Pat. Nos. 6,196,342 and 6,129,164 disclose reverse steering logic mechanisms that are coupled to and interact with a dual differential type of drive and steer transmission to cause the transmission to execute vehicle turns in the direction that the steering wheel is turned when operating in forward or reverse. These patents disclose the use of caster wheels, and do not disclose the use of steerable front wheels.
U.S. Pat. No. 6,921,109 discloses a reverse steering logic mechanism and a mechanism for providing “variable steering responsiveness.” It discloses using these mechanisms with the dual differential type transmission in U.S. Pat. No. 6,196,342.
U.S. Pat. No. 6,905,985 discloses a complicated system of linkages that purportedly provides for steering control of front steerable wheels and transmission-effected steering that rotates the rear wheels such that the vehicle turns in the direction that the steering wheel is turned when operating in forward or reverse. This patent discloses the use of this system in combination with a dual differential type transmission.
U.S. Pat. No. 6,152,248 discloses the use of a non-circular gear pair in the steering of a vehicle, but that gear pair does not control the turning of a non-driving wheel.